This invention relates to a pattern-matching joining machine, such as a sewing machine, for joining two sheets, such as cloths, each bearing the same patterns with the patterns matching.
Published Unexamined Japanese Patent Application No. S60-153896 (which corresponds to the U.S. Pat. No. 4,612,867, and the German Patent Application No. DE 33 46 163 C1) discloses a pattern-matching sewing machine of this type. In this machine, a photo-sensor is placed before the sewing point to generate intensity data representing the brightness of the patterns on the two cloths. The mismatch distance of the patterns on the two cloths is detected using the intensity data, and the relative feed amount of the two cloths is adjusted according to the mismatch distance to maintain the pattern match.
A problem is that there are patterns that cannot be correctly adjusted by using the brightness alone. An example is a pattern including a foreground pattern of chromatic color with a low brightness (e.g., dark green) on a background pattern of a bright achromatic color and a dark achromatic color (e.g., white and black). In this case, the conventional pattern sensing method cannot accurately recognize the low-brightness chromatic color (dark green) because it is obscured by the bright achromatic pattern.
Another such pattern that cannot be recognized by the prior art method is the pattern composed by superposition of two color patterns of similar brightnesses. An example is a blue foreground pattern with a red background pattern both having similar brightnesses. In this case, one of the patterns, say the red pattern, cannot be accurately discriminated from the other (blue). Placing a red filter in the optical path between the cloth and the sensor to selectively pass the red light would be one solution. But once a color filter is fixed to the red color, the pattern sensor could not cope with other patterns having various colors: that is, the variety of cloth-patterns that could be matched would be limited.